When a rotatable sample holder is used in conjunction with a relatively movable scanning device, it is known to scan around the holder in order to establish or confirm the radius of the holder and the co-ordinates of the origin of the holder. This information is used to interpret data produced during the scanning of an object located on the sample holder.
A problem with this method is that it assumes that the sample holder surfaces are square and that the axis of the circumferential surface of the sample holder remains co-linear with respect to its rotational axis along its length i.e. that the surface of the sample holder on which a sample is located is perpendicular to the rotational axis. Additionally, in order for this assumption to be treated as valid, the equipment must be manufactured to tight tolerances which increases the cost of the equipment.
An alternative scanning system uses Cartesian scanning in which case the sample holder is stationary during a scan. Traditionally it is assumed that the centre line of the sample holder is square to the axes of the scanning system however, for certain applications where it is important that the longitudinal axis of a sample is known this assumption may be invalid leading to errors.